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ok i am new to all of this so i know i ask some crazy questions please just bare with me.i would like to build a PI detector and i know i should start with the easiest to build.but what i would like to know is, is there a diy PI that i can build that will eliminate the ground signal?that would be my main concern.threshold,delay,sample, should all be in the mix as well.and if there was a disc. that would be nice as well but not a must have..like i said earlier my main concern would be ground elimination...i would like to thank everyone that has replied to my other posts without judgement.and thank you for all of your help it is what us new people need is good advice
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Hey! Welcome to the forum.
The sought after Holey Grail is a Discrimination system for the PI detector.
No one has been able to do it as of yet.
Some have a Elimination Provition but No True Discrimination.
By Elimination, you end up loosing everything below that setting.
Have you seen this?
http://www.deeptech-bg.com/Understan...I_Detector.htm -
i really just need the ground elimination. the discrimination would be nice but i dont have to have it.thanks for the linkComment
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The PI is immune to ground problems to the most parts!
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ok as i stated i am new but thats what i was told by someone else.he said thats what i need to hunt in this very bad ground. i just did'nt know any better. thank you for your helpComment
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The PI detector is a different animal then most detectors.
http://electronics.howstuffworks.com...-detector4.htmComment
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PI does a pretty good job of ignoring mineralization, but in the very worst ground you may need a PI that can ground balance. I don't know of a DIY design with a GB channel.
- CarlComment
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thank you carl.you might know the man i'm talking about he helped in the testing of the whites TDI. the ground i am refering to is the heavy red clay of northern VA. i just wanted to know if a GB could be adapted to any of the diy PI's. i live in Greenville NC and i do plan on trying my had at a detector i want to try yours as well as some others. i have a pcb for a XR71 on its way its the first i have orderedComment
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Originally posted by homefire View Post
In the link, there is one sentence describing the value of the coil damping resistor, that gives me headache:
If the resistor has too high a value, there will be some very minor oscillation, and if the resistor is too low in value, the spike voltage will take too much time dropping to the OV range.
If the resistor is "too" low in value, shouldn't it actually speed up the spike voltage dropping time?
Thanks,
MarkusComment
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Hi Digger,
I think the "Goldscan 4" or "GS4" has real ground balance and some limited discrimination. There is much info on this site if you do a search.
MarkComment
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"...i really just need the ground elimination..."
There is not such a detector in the known universe with the ability to eliminate (completely) the ground signals, PI or VLF, DIY or commercial.
The detectors simply eliminate the ground signals at some point, either these signals derive from mineralization or conductivity.
Minelab (GPX series) can do this, far better than the rest companies.
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The damping coefficient of a parallel RLC circuit is 1/(2RC), so low R results in longer damping. If you remove the damping resistor (R=∞) the PI coil will oscillate, which represents the fastest drop time.Originally posted by msysta View PostIn the link, there is one sentence describing the value of the coil damping resistor, that gives me headache:
If the resistor has too high a value, there will be some very minor oscillation, and if the resistor is too low in value, the spike voltage will take too much time dropping to the OV range.
If the resistor is "too" low in value, shouldn't it actually speed up the spike voltage dropping time?
Thanks,
Markus
- CarlComment
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Carl reinvents the "Sine induction"
Quote: "The damping coefficient of a parallel RLC circuit is 1/(2RC), so low R results in longer damping. If you remove the damping resistor (R=∞) the PI coil will oscillate, which represents the fastest drop time". Carl
Carl, you reinveted a special case of Pulse induction named "Sine induction":-). However your network "LC without damping" represents infinitely long drop time; it oscillates continuously :-)
The time domain's term "fast drop time" means "wide frequency band" in frequency domain. The "Sine induction" works with narrowest frequency band (about 4 to 16Hz).
In 11-17-2009 I published a simplified equivalent circuit diagram of PI TX Network and formula describing its critical damping condition. See the thread
Can the inductive time constant formula be applied to coil?
The circuit and the formula show that the network has two damping resistances: R and coil's "r".Comment
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GLOSSARY
Critical damping of TX Network means that the current through the TX coil stops maximal quickly without ringing. The above formula is valid for this case. In frequency domain this means widest frequency spectrum.
Critical damping of RX Network means that the voltage in the input of RX preamp stops maximal quickly without ringing. Above formula is not valid for this case becase the input resistance forms with coil resistance voltage divider.
FastCoil is a term introduced by J. Rogowski to indicate a condition that allows to shorten Sample delay (the time interval for taking the first sample). For this aim, the coil is made so as to have minimum stray capacitance and suitable low inductance. But this is not enough because the FastCoil is connected to a SlowNetwork having its stray capacitance :-(.
The Time domain's term FastNetwork means in Frequency domain wideband network having high upper limit frequency. This alows to wideband metal detector to analyze the range of frequencies that are lower than the networks resonant frequency. To calculate inductance, we need to know total networks stray capacitance and upper limit frequency.
The above equivalent circuit represents linear parameters, but at open switch there are in addition: an invisible nonlinear damping resistor, formed by the MOSFET, and an invisible Colpitts oscillator, again formed by the MOSFET.
DESCRIPTION
The attached below figure is more detailed equivalent circuit of PI TX than in the above figure. This is the SlowNetwork containing a FastCoil :-).
The FastCoil's parameters are enclosed in a brocken line rectangle. Added is a low ohm resistor Rm intended to monitor the drain current with the oscilloscope or to restrict the maximal value of the drain current. Added is a non-linear resistor Rz, expressing breakdown (avalanche) voltage of MOSFET, but it can be a real non-linear resistor type VDR or Zener diode for high voltage with aim to reduce the heat of the MOSFET at turn off process. Adding VDR or ZD is unnecessary because the released energy can not cause damage to the MOSFET, but would probably damages a BJT.Comment
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